1. Field of the Invention
The present invention relates to a lithographic apparatus having a debris-mitigation system, a source for producing EUV radiation having a debris mitigation system, and a method for mitigating debris.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
During use of at least a part of the lithographic apparatus, debris particles may be formed within the lithographic apparatus, for example, when radiation sensitive material is removed from the substrate. However, this is not the only process which may give rise to the production of debris particles. For the generation of EUV radiation, usually an object (or gas) is heated by means of high current pinching or by a laser source of high energy density, for example, of at least 1011 W/cm2, to create a plasma. EUV radiation is produced and undesired particles are released. Also, during production of EUV radiation, which, within the lithographic apparatus is used for providing a projection beam of radiation, debris particles may be produced as a by-product.
The term “debris particles” is meant to encompass particles which are thus formed as a by-product and which need to be mitigated. Debris particles may comprise electrons, ionized particles or neutral particles. The debris particles may have a size in a range varying from the size of an electron towards the size of a micro sized particle. The particles may be part of a plasma and may, within the plasma-environment, comprise fast or slowly moving particles.
These debris particles may have a detrimental effect on the performance of the lithographic apparatus as these debris particles may collect on, for example, optical elements. The debris particles present on these optical elements may hinder the reflection or transmission of the radiation beam. Debris particles with relatively high kinetic energy may even cause serious damage to the apparatus by, for example, etching the optical elements which are, incidentally, very expensive and very crucial parts of the apparatus. Particles present on, or etching surfaces of optical elements may also cause the radiation to be directed in an inaccurate manner, leading to poorly defined features produced on a substrate exposed to the radiation, hence, a need for mitigating debris particles.
The term “mitigating” is meant to encompass any manipulation of the debris particles which may lead to, at least, a reduction of the detrimental effect on the performance of the lithographic apparatus, a reduction of the damage to the lithographic apparatus, or both.
The invention is in particular related to a lithographic apparatus comprising: an illumination system for providing a projection beam of radiation; a support structure for supporting a patterning device, the patterning device serving to impart the projection beam with a pattern in its cross-section; a substrate table for holding a substrate; a projection system for projecting the patterned beam onto a target portion of the substrate; and a debris-mitigation system for mitigating debris particles which are formed during use of at least a part of the lithographic apparatus.
Such a lithographic apparatus is described in WO 99/42904. The debris-mitigation system known from this prior art comprises a filter, in particular a so-called foil trap, which may comprise a plurality of foils or plates which capture the debris particles. The plates or foils are as much as possible oriented parallel to a radiation path so that radiation will pass through the filter.
Although the foil trap is useful, a lithographic apparatus with an additional or alternative debris-mitigation systems is required to, for example, improve the efficiency of the debris-mitigation system, the flexibility in application of a debris-mitigation system and/or allow for more freedom in the design of a lithographic apparatus. It should be understood that charged particles may also be formed from neutrals which are being ionized during mitigation.